Machinist question

[Don Cowles]

I have dug myself into a hole. I have committed a cardinal sin: I have 5 knives to deliver wednesday night, and I did not make a prototype. They are my basic straight blade, but the carbon fiber scales will be held on with 4-40 screws, for which the blades were tapped before heat treat.

THE PROBLEM: Although there does not appear to be any scale in the threaded holes, I can only get a screw (or a tap) part way into the holes- so I am assuming they shrunk during heat treat.

QUESTION ONE: can I fix it, and how?

QUESTION TWO: what can I do to prevent this in the future?

Many thanks -

[Tom Anderson]

I wouldn't try tapping, because the tap would probably just break.

Localized bead blasting might remove enough material that the screws will eventually go in.

[DC KNIVES]

Just a guess, but would spot annealling work? I have done this on carbon steel,but not stainless.Good luck Don,Dave.

[Geno]

Don, seal up the holes with vinegar overnight, and let it eat that black off the iside(it is a microscopic scale).
Come back later and blow it out with high air pressure real good, then tap it using oil. It should come right out, but don't force it.
Try a used tap first if you have any old ones.
Don't use a tapping machine now either, do it by hand so you can feel it. Hope it gets you out in time.
Be blessed.

[neil charity]

I've had a similar problem in the past Don, I get around it by running the screws I'm going to use in the final setup before the blades are heat treated. Seems to work OK however Gene's solution should work also.

Regards from Australia.

[Don Cowles]

What's baffling here is that I did exactly what Neil suggests - and all the screws went into all the holes. After HT, they were all bead blasted. I have spot annealed, and broke off a tap in one of the blades.

Next step is the vinegar, then the trash can. Thanks guys.

[Jerry Shorter]

Don,
Two things you can do in the future. Use a (.003 ) larger tap drill, #43 for 4-40 . For the purpose of knife handles a 60% thread will work fine.( tap drill charts are set for 75% thread) 2. Get an oversize tap. Over sized taps are Ideal for steel that must be tapped befor heat treat. Spiral point (gun) taps are much stronger than spiral flute taps. the best way to use gun taps is with a tapping head.
Hope this helps in the future
Jerry

[SharpByCoop]

I like Gene's answer. Going further, since you are getting down to the wire, I'd try sealing some acid (ferric?) in that hole for a short while. I can't imaging it not working.

At my shop I also get success with buggered-up threads by using a fine lapping compound on both the thread and the bores. In and out, in and out, like a tap. That works, albeit slowly.

Keep us posted.

Coop

[Colin KC]

Another option (all be it an extreme & decidedly fiddly one) is to tighten up a die real good & cut the thread on the screws a bit more. (just make sure you use blue loctite )

[Don Cowles]

Thanks for the suggestions, guys. Jerry, good tip on the oversize tap drill/gun tap. I will call Brownells today!

[Rob Frink]

Don,

In the future, I think the solution may be simple. It sounds like you may be using the wrong "thread class" tap...or "h-limit". Taps have tolerances and are denoted with : H1, H2,...H6. H1 is the smallest tap size and H6 is the largest size for a given thread.

So look at the tap that you are using....look for the "H" number on the shank and make a note of it...then order a larger "H" next time. For example, if you are using an H2 tap.....move up to an H4,5 or 6. Premium taps from from industrial suppliers are offered this way. The purpose of the sizes is to acount for plating build-up.

I'm very curious...so if you look...please let me know what you find.

Good luck,
Rob

[Jerry Shorter]

Don, What Rob says is true . the oversized tap that I am referring to is grade H11. Most taps that you buy are grade H3 for inch sized taps and H4 for metric taps. ( grade refers to size limit). When using acid to etch out the threads check it often because it degrads the entire thread form including the thread angle. and you will get a very sloppy fit. If not extremly sloppy the blue loctite will make it better. but loctite is no replacement for threads.If it does get extremly sloppy I dought if you would be happy with the results.
Jerry

correction: 4-40 taps are generally grade H2

P.s. if Brownell's doesn't have them give McMaster Carr a call or check their web site, they stock them #2612A11 for a 4-40 oversized tap.
Just found this excelent information
http://www.morsecuttingtools.com/ref...gineering.html

[Don Cowles]

Guys, what blows me away is that the screws fit the holes just fine before HT. There is no visible scale on the threads, either.

[Jerry Shorter]

Don, it very common for screw threads to distort during heat treat, Using an oversized taps is common practice. the threads will still distort but the diference in size makes up for the distortion.
Jerry
......a little later, I was just looking in the Mcmaster Carr catalogue and I see where they have heavy duty taps that will tap hardened steel (30-48C) Your tang may be quite a bit harder than 48C but one of these might work to clean up the threads. They are pricey but might be worth it to save the blades. 4-40 part #25215A51 about $14.00

[Don Cowles]

A gunsmith in the neighborhood cleaned them up with a TI coated cobalt tap, so for a $20 bill, I am out of the woods for now. I will surely avoid this problem in the future by using oversize taps!!@#$%!!